The formation and maintenance of metazoan tissues are orchestrated by a complex succession of events that include the maturation, proliferation, migration and adhesion of cells. At their core, these processes all depend on the outputs of signal transduction pathways that are controlled by the antagonistic activities of tyrosine kinases and tyrosine phosphatases. While the spotlight often shines on tyrosine kinases, tyrosine phosphatases, and especially receptor protein tyrosine phosphatases (RPTPs), remain understudied. These molecules mediate cell-cell or cell-matrix adhesion, but the exact identities of many of the RPTP extracellular ligands are unclear. Furthermore, the physiological roles of these interactions, beyond those described in neural tissues, are ill-defined. Broadly, we seek to investigate the non-neuronal functions of RPTPs to ultimately understand how the inappropriate regulation of RPTP function may result in disease. Our recent studies have focused on the role of RTRPs in the musculature, whereby we have uncovered an unexpected role for the evolutionarily conserved RPTP Drosophila lar (Dlar). Signaling components that regulate Dlar function in muscle tissue - including the signals that initiate and respond to Dlar phosphatase activity - have remained elusive. In this application, we will expand upon our novel preliminary data showing that: (1) loss of Dlar specifically in the muscle is lethal, and (2) knockdown of Dlar leads to the mislocalization of integrin subunits at muscle-specific costameres, protein complexes that are essential for the transmission of contractile forces. These findings have led us to the overall hypothesis that integrins and Dlar interact physically to maintain the cytoskeletal architecture in the musculature. Thus, we propose to use a powerful combination of biochemical, structural, and genetic approaches to complete the following specific aims: (1) to identify the molecular basis and evolutionary conservation for the genetic interaction between Dlar and integrins and (2) to utilize the musculature as an in vivo model to determine the role of Dlar in integrin function at muscle costameres. Importantly, the costamere complex and other proteins necessary for muscle contraction and homeostasis are well conserved throughout the animal kingdom. This argues that our proposed work in the Drosophila musculature will be relevant to mammalian muscle tissue homeostasis. Our contribution will be significant because it will help define the fundamental mechanisms that underpin maintenance of the muscle cytoskeleton in actively contracting muscles, broaden our understanding of the physiological functions of RPTPs and provide insight into other biological processes where RPTPs and integrins may function synergistically.